A maraging steel generally has a very high tensile strength of about 2000 MPa, and thus it has been used for members required to have high strength in various applications, such as rocket parts, centrifugal separator parts, aircraft parts, continuously variable transmission parts of automobile engines, or dies. A typical composition of the maraging steel contains 18% Ni, 8% Co, 5% Mo, 0.4% Ti, 0.1% Al and the balance of Fe.
The maraging steel contains appropriate amounts of Co, Mo and Ti as hardening elements, and can obtain high strength by precipitating intermetallic compounds such as Ni3Mo, Ni3Ti or Fe2Mo through aging treatment. In particular, it is an important requirement to have fatigue strength particularly in a high cycle region for a steel strip used for continuously variable transmission parts of automobile engines. Thus, it is required to make nonmetallic inclusions such as TiN, which are included in the maraging steel having the high strength, fine as far as possible. Moreover, the maraging steel has been used by subjecting it to nitriding treatment to form a nitrided layer on its surface to improve fatigue strength.
For example, JP-A-2004-514056 (Patent Literature 1), JP-A-2001-240943 (Patent Literature 2) and JP-A-2002-167652 (Patent Literature 3) proposed improved alloys for avoiding decrease in fatigue strength, which occurs due to a nonmetallic inclusion as a starting point, for metallic belts used for continuously variable transmissions of automobile engines.
The applicant also has proposed improved alloys for avoiding the decrease in fatigue strength occurring due to a nonmetallic inclusion as a starting point, which alloy contains reduced Ti content of 0.1 mass % or less so as to substantially eliminate the inclusions such as TiN, in JP-A-2008-088540 (Patent Literature 4), JP-A-2007-186780 (Patent Literature 5) and WO2009-008071 (Patent Literature 6).
Moreover, JP-A-2008-185183 (Patent Literature 7) has proposed a method for producing a maraging steel strip having high fatigue strength, in which the maraging steels described in the above Patent Literatures 4 to 6 are heated and maintained in a gas atmosphere containing fluorine compounds to remove an oxide film from their surface, and then are subjected to nitriding treatment at a temperature of 400 to 500° C. in a nitriding gas that is controlled to have NH3/H2 gas composition ratio from 1 to 3.